Qualitative or quantitative observation of the pupilary reflex of human subjects, i.e. the variation of the pupil size caused by external stimuli, is of clinical significance because, from a normal or abnormal behavior of this reflex, conclusions can be drawn on the functional state of the brain structures and nerve tracks that participate in stimulus transmission from the locus of stimulation to the muscle bundles regulating the size of the pupil.
Stimuli, apart from optical, may be also acoustical, tactile, thermal or other, thus providing many possibilities to test parts of the neural system. This sort of functional test is most valuable for the clinician when he has to deal with non-collaborating subjects, as, for example, unconscious persons or infants.
Methods for measuring the pupillary reflex of collaborating subjects are known. The subject is told to fixate a target with open eye and the pupil is viewed for example by a cinema or television camera before, during and after application of the stimulus or the eye is illuminated with red or infrared light, and the pupil and the iris are imaged onto a photocell. All known methods are, however, applicable only with difficulty to non-collaborating subjects.
In the case considered here it has to be supposed that the subject, in general, will hold his eyes closed. Nevertheless, his eyeballs may move in an arbitrary and not foreseeable manner. Even when, in such cases, the eyelids were artificially held open, a reliable measurement of the pupil size by known methods would be rendered considerably less accurate, since the pupil would appear distorted by perspective as a consequence of arbitrary eye movements.